The present invention relates to the formation of a nonwoven web by melt extrusion. More particularly, the present invention relates to a method of imparting delayed wettability to a nonwoven web prepared by melt extrusion.
Traditional melt-extrusion processes for the formation of a nonwoven web from a thermoplastic polymer typically involve melting the thermoplastic polymer, extruding the molten polymer through a plurality of orifices to form a plurality of threadlines or filaments, attenuating the filaments by entrainment in a rapidly moving first stream of gas, cooling the filaments with a second stream of gas, and randomly depositing the attenuated filaments, or fibers, on a moving foraminous surface. The most common and well known of these processes are meltblowing, coforming, and spunbonding. The nonwoven webs obtained by these processes are widely used in a variety of products, but especially in such disposable absorbent products as diapers, incontinent products, feminine care products, such as tampons and sanitary napkins, wipes, sterilization wraps, surgical drapes and related materials, hospital gowns, shoe covers, and the like, to name but a few.
Meltblowing references include, by way of example, U.S. Pat. Nos. 3,016,599 to R. W. Perry, Jr., 3,704,198 to J. S. Prentice, 3,755,527 to J. P. Keller et al., 3,849,241 to R. R. Butin et al., 3,978,185 to R. R. Butin et al., and 4,663,220 to T. J. Wisneski et al. See, also, V. A. Wente, "Superfine Thermoplastic Fibers", Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346 (1956); V. A. Wente et al., "Manufacture of Superfine Organic Fibers", Navy Research Laboratory, Washington, D.C., NRL Report 4364 (111437), dated May 25, 1954, United States Department of Commerce, Office of Technical Services; and Robert R. Butin and Dwight T. Lohkamp, "Melt Blowing-A One-Step Web Process for New Nonwoven Products", Journal of the Technical Association of the Pulp and Paper Industry, Vol. 56, No. 4, pp. 74-77 (1973).
Coforming references (i.e., references disclosing a meltblowing process in which fibers or particles are comingled with the meltblown fibers as they are formed) include U.S. Pat. Nos. 4,100,324 to R. A. Anderson et al. and 4,118,531 to E. R. Hauser.
Finally, spunbonding references include, among others, U.S. Pat. Nos. 3,341,394 to Kinney, 3,655,862 to Dorschner et al., 3,692,618 to Dorschner et al., 3,705,068 to Dobo et al., 3,802,817 to Matsuki et al., 3,853,651 to Porte, 4,064,605 to Akiyama et al., 4,091,140 to Harmon, 4,100,319 to Schwartz, 4,340,563 to Appel and Morman, 4,405,297 to Appel and Morman, 4,434,204 to Hartman et al., 4,627,811 to Greiser and Wagner, and 4,644,045 to Fowells.
The polymers most often used in the foregoing processes, particularly for the types of products mentioned, are polyolefins. These polymers are naturally hydrophobic, which often is an undesirable characteristic.
A significant improvement over previously known methods of imparting hydrophilicity to otherwise hydrophobic polymers is described in U.S. Pat. No. 4,923,914 to Nohr et al., which patent is incorporated herein by reference. The patent describes a surface-segregatable, melt-extrudable thermoplastic composition which comprises at least one thermoplastic polymer and at least one defined additive. The most preferred additives are polysiloxane polyethers.
Upon being melt-extruded, the compositions of U.S. Pat. No. 4,923,914 result in fibers having a differential, increasing concentration of the additive from the centers to the surfaces thereof, such that the concentration of additive toward the surface of each fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present. The additive forms an emulsion with the polymer at melt extrusion temperatures, under which conditions the additive and the polymer form a metastable solution. As the temperature of the newly formed fiber drops below melt extrusion temperatures, the additive becomes significantly less compatible with the polymer. Concurrent with this marked change in compatibility, the polymer begins to solidify. Both factors contribute to the rapid migration or segregation of the additive toward the surface which takes place in a controllable manner.
Web integrity sometimes is a problem with the compositions of U.S. Pat. No. 4,923,914. When the additive is a siloxane-containing compound and the desired characteristic is water-wettability, the resulting nonwoven webs can lack integrity upon their formation because of the presence of additive on the surfaces of the fibers. The additive sometimes interferes with the fiber-to-fiber bonding upon which web integrity relies, especially at additive levels of about 1.5 weight percent or higher. In such circumstances, the additive also has a tendency to accumulate over time on the forming wire.
This problem of poor web integrity in nonwoven webs prepared such processes as meltblowing, coforming, and spunbonding can be rectified by instituting process changes. Alternatively, wettability can be delayed as described in Application Ser. No. 07/566,938, entitled METHOD OF PREPARING A NONWOVEN WEB HAVING DELAYED WETTABILITY and filed on Aug. 13, 1990 in the names of Ronald S. Nohr and J. Gavin MacDonald. The delay in wettability results from the use of a trisiloxane polyether having the general formula, ##STR1## in which: (a) R.sup.1 -R.sup.7 are independently selected monovalent C.sup.1 -C.sup.3 alkyl groups;
(b) R.sup.8 is hydrogen or a monovalent C.sup.1 -C.sup.3 alkyl group; PA1 (c) m represents an integer of from 0 to about 5; PA1 (d) n represents an integer of from 3 to about 8; PA1 (e) the molecular weight is from about 350 to about 700; PA1 (f) the polydispersity is from about 1.0 to about 1.3; and PA1 (g) the trisiloxane polyether is present in an amount of from about 0.5 to about 1.75 percent by weight, based on the amount of thermoplastic polymer, which amount, if homogeneously distributed throughout the polyolefin, is not sufficient to render the polyolefin wettable by water. PA1 (b) R.sub.10 is either hydrogen or a monovalent C.sub.1 -C.sub.3 alkyl group; PA1 (c) m represents an integer of from 1 to 3; PA1 (d) n represents an integer of from 0 to about 5; PA1 (e) p represents an integer of from 0 to about 5; PA1 (f) x represents an integer of from 1 to about 10; PA1 (g) y represents an integer of from 0 to about 5; PA1 (h) the ratio of x to y is equal to or greater than 2; PA1 (i) said additive has a molecular weight of from about 700 to about 1,300; PA1 (j) said additive has a polydispersity of from about 1.3 to about 3.0; and PA1 (k) said additive is present in an amount of from about 1.8 to about 3.5 percent by weight, based on the amount of thermoplastic polyolefin; PA1 (b) R.sub.12 and R.sub.13 are independently selected monovalent C.sub.1 -C.sub.3 alkyl groups; PA1 (c) q represents an integer from about 1 to about 12; PA1 (d) x represents an integer from 1 to about 10; PA1 (e) y represents an integer from 0 to about 5; and PA1 (f) the ratio of x to y is equal to or greater than 2; PA1 (g) said additive has a molecular weight of from about 700 to about 1,300; PA1 (h) said additive has a polydispersity of from about 1.3 to about 3.0; and PA1 (i) is present in an amount of from about 1.8 to about 3.5 percent by weight, based on the amount of thermoplastic polyolefin; and PA1 (a) is insoluble in the polymer at both ambient and melt-extrusion temperatures; PA1 (b) is present in an amount of from about 0.1 to about 1 percent, based on the weight of said thermoplastic composition; PA1 (c) has a surface area of from about 50 to about 1,000 m.sup.2 ; and PA1 (d) is capable of being at least partially coated by said additive.
A method of increasing the wettability delay period of the nonwoven webs obtained in cross-referenced Application Ser. No. 07/566,938 is disclosed in cross-referenced Application Ser. No. 07/488,344. Such increase in the delay period results from including in the thermoplastic composition, in addition to the defined trisiloxane polyether, from about 0.1 to about 6 percent by weight, based on the amount of thermoplastic polymer, of at least one material having the capacity to increase the delay period for up to about two weeks. The preferred material for increasing the delay period is a phthalocyanine dye.
Notwithstanding the teachings of Application Ser. Nos. 07/566,938 and 07/488,344, there still is need for a method of further delaying the development of wettability in nonwoven webs in a more controlled manner. In addition, there is a need to provide greater time control and to extend the wettability delay period. Furthermore, a method is needed which avoids coloring the fibers as with the use of a phthalocyanine dye.